The propagation time of a logic circuit, for example, a logic gate or a CMOS inverter, is the time between the occurrence of a transition at the input of the logic circuit and the occurrence of the corresponding transition at the output of the logic circuit. This propagation time generally varies with temperature. This results in consequences for certain parameters of certain components, for example ring oscillators, using CMOS inverters as CMOS logic circuits.
A ring oscillator comprises an odd number of looped CMOS inverters.
Furthermore, the variation in the propagation time of the CMOS inverters as a function of temperature induces a variation in the frequency of the oscillator as a function of temperature.
In general, current sources which deliver a current proportional to absolute temperature (known to the person skilled in the art by the acronym PTAT) or a current inversely proportional to absolute temperature (known to the person skilled in the art by the acronym CTAT: Complementary to Absolute Temperature) are used in order to supply such components. However, it is not then possible to obtain temperature compensation with a single current source.
One known solution for carrying out temperature compensation of certain parameters, for example the frequency of an oscillator, consists in using a combination of a plurality of current sources, for example one source of the PTAT type and one source of the CTAT type, so as to obtain a supply current which is very insensitive to temperature variations. However, such solutions lead to devices which have significant bulk and consequent consumption.